Disease in plants is caused by biotic and abiotic causes. Biotic causes include fungi, viruses, bacteria, and nematodes. Of these, fungi are the most frequent causative agent of disease on plants. Abiotic causes of disease in plants include extremes of temperature, water, oxygen, soil pH, plus nutrient-element deficiencies and imbalances, excess heavy metals, and air pollution.
A host of cellular processes enables plants to defend themselves from disease caused by pathogenic agents. These processes apparently form an integrated set of resistance mechanisms that is activated by initial infection and then limits further spread of the invading pathogenic microorganism. This limitation of the pathogen intruder is frequently accomplished by localized containment of the intruder following a coordinated complex defense response.
Subsequent to recognition of a potentially pathogenic microbe, plants can activate an array of biochemical responses. Generally, the plant responds by inducing several local responses in the cells immediately surrounding the infection site. The most common resistance response observed in both nonhost and race-specific interactions is termed the "hypersensitive response" (HR). In the hypersensitive response, cells contacted by the pathogen, and often neighboring cells, rapidly collapse and dry in a necrotic fleck. Other responses include the deposition of callose, the physical thickening of cell walls by lignification, and the synthesis of various antibiotic small molecules and proteins. Genetic factors in both the host and the pathogen determine the specificity of these local responses, which can be very effective in limiting the spread of infection.
The hypersensitive response in many plant-pathogen interactions is specified by and dependent on the presence of two complementary genes, one from the host and one from the pathogen. These complementary genes are the resistance (R) gene in the plant and a corresponding avirulence (avr) gene in the pathogen. The interaction of the genes is associated with the rapid, localized cell death of the hypersensitive response. R genes that respond to specific bacterial, fungal, or viral pathogens, have been isolated from a variety of plant species and several appear to encode cytoplasmic proteins.
The resistance gene in the plant and the avirulence gene in the pathogen often conform to a gene-for-gene relationship. That is, resistance to a pathogen is only observed when the pathogen carries a specific avirulence gene and the plant carries a corresponding or complementing resistance gene. Because avr-R gene-for-gene relationships are observed in many plant-pathogen systems and are accompanied by a characteristic set of defense responses, a common molecular mechanism underlying avr-R gene mediated resistance has been postulated. A simple model which has been proposed is that pathogen avr genes directly or indirectly generate a specific molecular signal (ligand) that is recognized by cognate receptors encoded by plant R genes.
Both plant resistance genes and corresponding pathogen avirulence genes have been cloned. The plant kingdom contains thousands of R genes with specific specificities for viral, bacterial, fungal, or nematode pathogens. Although there are differences in the defense responses induced during different plant-pathogen interactions, some common themes are apparent among R gene-mediated defenses. The function of a given R gene is dependent on the genotype of the pathogen. Plant pathogens produce a diversity of potential signals, and in a fashion analogous to the production of antigens by mammalian pathogens, some of these signals are detectable by some plants.
The avirulence gene causes the pathogen to produce a signal that triggers a strong defense response in a plant with the appropriate R gene. However, expressing an avirulence gene does not stop the pathogen from being virulent on hosts that lack the corresponding R gene. A single plant can have many R genes, and a pathogen can have many avr genes.
Monogenic resistance mediated by recessive (mlo) alleles of the Mlo locus is different. It differs from race-specific incompatibility to single pathogen strains in that it is believed to confer a broad spectrum resistance to almost all known isolates of the fungal pathogen, and the resistance is apparently durable in the field despite extensive cultivation. Further, mlo resistance alleles have been obtained by mutagen treatment of susceptible wild-type Mlo varieties. These mlo plants exhibit a spontaneous leaf cell death phenotype under pathogen-free or even axenic conditions.
As noted, among the causative agents of infectious disease of crop plants, the phytopathogenic fungi play the dominant role. Phytopathogenic fungi cause devastating epidemics, as well as causing significant annual crop yield losses. All of the approximately 300,000 species of flowering plants are attacked by pathogenic fungi. However, a single plant species can be host to only a few fungal species, and similarly, most fungi usually have a limited host range.
Plant disease outbreaks have resulted in catastrophic crop failures that have triggered famines and caused major social change. Generally, the best strategy for plant disease control is to use resistant cultivars selected or developed by plant breeders for this purpose. However, the potential for serious crop disease epidemics persists today, as evidenced by outbreaks of the Victoria blight of oats and southern corn leaf blight.
Accordingly, molecular methods are needed to supplement traditional breeding methods to protect plants from pathogen attack. Particularly, methods are needed for broad spectrum resistance to pathogens.